| dc.description.abstract | The cathode of Li-ion battery after charge-discharged can produce a special structure named solid electrolyte interface (solid electrolyte interface, SEI), which helps lithium ion transmission during charge and discharge process, and improves the electrode stability. Charge capacity and safety can also be improved by improving SEI structure. The research presents two approaches to the improvement of SEI structure with the aim to produce high performance and safe lithium battery. This study is divided into two parts. The first part describes the modification of SEI through the addition of conducting polymer, which enhances both the C-rate capacity and cycle life. After screening several monomers, the thiophene monomer derivatives: 3,4-Ethylenedioxythiophene (EDOT) is found to be most effective for this purpose after electrochemical polymerization on the cathode surface. The conductive polymer is composed of electronic conducting main chain and ion conductive side-chain, enabling an efficient charge transfer on the interface of the LiFePO4 cathode particles. The addition, EDOT has effectively improved the C-Rate capability and life cycle. The improvement is found to increase with EDOT concentration but optimized at certain threshold. In presence of excess EDOT, longer activation cycle (during which polymerization is achieved) will be required to completely polymerize EDOT, and the interface may be too thick whcih blocks the litium insertion. In this system, 0.03M is the optimized concentration.
The second part of the study is to improve lithium battery safety feature with the addition of functional molecules in the electrolytes. LiCoO2 is the most widely used cathode material in commercial lithium ion batteries, but the safety remains an issue which urgently needing improvement. In this research, barbituric acid (BTA) and its derivatives as well as conducting polymer were added to the electrolyte to improve lithium battery safety. The delay of exothermic temperature can be observed by differential scanning thermal calorimetry (DSC). Carefully balancing the component composition, it is found the battery performance and safety features can both be enhanced. Conventional safety technology uses flame retardants to reduce electrolyte flammability, temperature control is not satisfactory, and the charge capacity usually suffers greatly. In contrast, present approach achieved the thermal stability while still maintaining the charge capacity and long cycle life.
| en_US |